1. Field of the Invention
The present invention relates to a technique for driving a multiphase direct current (DC) motor.
2. Description of the Related Art
In portable Compact Disk (CD) apparatuses and electronic apparatuses using disk-type media such as Digital Versatile Discs (DVDs), brushless DC motors (spindle motors) are used for rotating the disks. The brushless DC motor generally comprises: a rotor provided with a permanent magnet; and a stator provided with a plurality of coils connected together in star connection. In the motor, the coils are energized by controlling a current supplied thereto such that the rotor is relatively rotated with respect to the stator. The brushless DC motor is generally provided with a sensor such as a Hall element and an optical encoder to detest a rotational position of the rotor. In accordance with the position detected by the sensor, a current supplied to each phase coil is switched such that the rotor is offered with an adequate torque.
In order to suppress a noise emanating from a motor, there has been presented a technique in which a conduction period of each phase coil (U-phase, V-phase, W-phase) of the motor is subjected to switching by using a pulse width modulated signal, the pulse width of which varies in a sine wave pattern, so that a current flowing through the coil of the motor (also referred to as a coil current) is slowly varied in a sine wave pattern. Alternatively, a trapezoidal waveform is sometimes used instead of a sine wave.
[Patent Document 1] Japanese Patent Application Laid-open No. Hei 3-207250
[Patent Document 2] Japanese Patent Application Laid-open No. Hei 10-243685
[Patent Document 3] Japanese Patent Application Laid-open No. Hei 11-75388
[Patent Document 4] Japanese Patent Application Laid-open No. Hei 8-331887
The present inventor has studied a motor drive circuit that varies the coil current of a motor in a sine wave pattern. As a result, the inventor has recognized the following challenge.
The inventor's recognition is that, even if a pulse width of a pulse width modulated signal is varied according to an ideal sine wave, the coil current does not necessarily have the ideal sine wave. This is because coil inductance of the motor has a variation dependent on its type and individual variation. In other words, it can be said that conventional motor drive techniques admit of further enhanced silence realized by bringing the waveform of the coil current closer to a sine wave.
In addition, inexpensive motors particularly have such large variations that the motors are difficult to be used as a spindle motor in a hard disk apparatus in which silence is needed, leading to the situation where expensive motors have to be used. If silence can be enhanced by improving the motor drive circuit, inexpensive motors can be used in wider areas.